Exemplary embodiments of the invention generally relate to rotary wing aircrafts, and more particularly, to a control system for pitching the blades of a rotor of a rotary wing aircraft.
Control of a rotary wing aircraft is affected by varying the pitch of the rotor blades individually as the rotor rotates and by varying the pitch of all of the blades together. These are known respectively as cyclic and collective pitch control. Blade pitch control of a rotary wing aircraft main rotor is commonly achieved through a swashplate.
The swashplate is typically concentrically mounted about the rotor shaft. The swashplate generally includes two rings connected by a series of bearings with one ring connected to the airframe (stationary swashplate) and the other ring connected to the rotor hub (rotating swashplate). The rotating ring is connected to the rotor hub through a pivoted link device typically referred to as “scissors”, with the static ring similarly connected to the airframe. The rotating swashplate rotates relative the stationary swashplate. Apart from rotary motion, the stationary and rotating swashplate otherwise move as a unitary component. Cyclic control is achieved by tilting the swashplate relative to a rotor shaft and collective control is achieved by translating the swashplate along the rotor shaft.
Pitch control rods mounted between the main rotor blades and the rotating swashplate mechanically link the rotating swashplate to each individual main rotor blade. Main rotor servos extend between and attach to the stationary swashplate and the airframe. Displacement of the main rotor servos results in displacement of the stationary swashplate. Displacement of the stationary swashplate results in displacement of the rotating swashplate. Displacement of the rotating swashplate results in displacement of pitch control rods and therefore each individual main rotor blade. Hence, by actuating selected main rotor servos, collective and cyclic commands are transferred to the rotor head as vertical and/or tilting displacement of the swashplates resulting in pitch control of the main rotor blades.
The swashplate and its associated linkages require a considerable amount of space, add to the aerodynamic drag of the aircraft, and account for a significant amount of gross weight. Due to their complexity and flight critical nature, the swashplate systems require regular and costly maintenance and inspection. Additionally, control inputs from swashplates are limited to collective and cyclic, which limit the resulting blade motion to steady and once per revolution rotation. Blade motions at higher harmonic frequencies have shown potential aircraft benefits such as improved performance and vibration. Thus, there is a continuing effort to improve blade pitch control for rotor systems of a rotary wing aircraft.